Electrostatic spray painting of plastics is of interest commercially due to the increase in transfer efficiency when painting plastics using electrostatic deposition and the corresponding decrease in emissions since less sprayed paint is used to provide adequate coverage to the plastic part. The electrostatic painting process requires that an electrostatic field suitable for the deposition of charged paint particles be established at the article being painted. When the article itself is conductive, it is typically electrically grounded to serve as an electrode for the electrostatic field. When the article is not conductive, conductive coatings, either high molecular weight (a primer) or low molecular weight, are used on the surface to be painted to increase the conductivity of the surface sufficiently to serve as such an electrode.
In the case of a conductive primer, the primer may serve other purposes than solely to provide a conductive surface. Primers may yield a better surface for painting, prevent solvent penetration into the plastic surface being painted, and/or a variety of other functions. In many cases, conductivity is imparted to the primer at the expense of these other properties. Thus the formulation of primers is restricted by the need to provide conductivity.
The low molecular weight coatings are organic salts or mixtures of organic and ionic species having the characteristics of being able to be adsorbed on the surface of the plastic and having the ionic species positioned such that a conductive surface is generated. Problems with appearance (the coating shows through the color coat or in some way alters the color, uniformity, distinction of image or other appearance characteristic of the paint and adhesion (the coating interferes with the interaction between the surface of the plastic and the paint after cure) are possible. In the long term, poor adhesion can lead to a separation of the coating film from the plastic.
To avoid using such conductive coatings on the surface to be painted, it has previously been proposed to locate a grounded metal backing plate at the back of the plastic panel to provide a terminus for an electrostatic paint deposition field. While this may be useful where the appearance and uniformity of the paint is not critical, such backing plates do not provide either intimate contact with the panel or adequate conformation to the article for a suitable field electrode (resulting in nonuniform paint distribution), and create the expense of a large inventory of specially shaped plates for each panel shape and the need to attach and remove a plate to the panels.
It has further been proposed, as set forth in U.S. Pat. No. 3,741,793 to Simmons, to coat the rear of the part with water or other conductive liquid which is grounded to serve as an electrode for electrostatic deposition on the front of the part. In particular, Simmons introduces a vapor to the inside of a hollow article which is cool enough to cause condensation of the vapor onto the inner wall. There is no indication how this process might be extended to an open structure such as a panel or whether it produces a coating of sufficient conductivity and continuity to result in a high quality coating meeting rigorous appearance specifications. In any event, it is expected that the use of such a liquid electrode on an article, however applied, is limited to special paint jobs such as painting bottles. In an environment such as painting automotive parts it is the practice to mount the parts onto carriers, wash the parts and then paint them; a liquid conductive coating on the part would be washed off before the painting step. Additionally, the parts may experience high temperatures and time delays after a conductive liquid coating is applied and before painting, thereby allowing the liquid coating to dissipate before it serves its function.